(1) Field of the Invention
This invention relates to bearing, and more particularly to bearing support structures useful in turbine engines.
(2) Description of the Related Art
Gas turbine engines of the type used for aircraft propulsion have a rotor that typically includes a fan having an array of radially extending fan blades, a compressor, a turbine and a shaft for transferring power and rotary motion from the turbine to the compressor and fan. The rotor is radially and longitudinally supported on a nonrotatable support frame by two or more longitudinally spaced-apart bearings, each enclosed within a sealed bearing compartment. Under normal operating conditions, the rotor has a center of gravity that is radially coincident with a central axis of the engine passing through the centers of the bearings. The rotor also has key natural vibratory frequencies which, by design, are higher than the rotor's maximum rotational frequency. By way of a non-limiting example, a turbofan engine having a cruising speed of 2,000-2,500 rpm and a full thrust/take-off speed of about 3,000 rpm may have key natural frequencies advantageously at least about 10% higher than the full thrust speed (e.g., about 3,300 rpm or 50.5 Hz).
During engine operation, a fan blade or a fragment thereof may become separated from the remainder of the fan (a so-called "blade-off" or "blade-out" event) so that the fan's center of gravity (center of mass) is displaced from the central axis. The center of gravity of the entire rotor is displaced in a similar radial direction but by a smaller amount. At least initially, the bearings constrain the rotor radially, so that it continues to rotate about the central axis rather than about an axis passing through the displaced center of gravity. The rotation of the displaced center of gravity about the central axis provides a forcing function which may excite one or more modes of oscillation of the rotor. At rotation speeds well below resonance, the imbalance produces local compression on the bearings generally in-phase with the displaced center of gravity. Approaching resonance there is an angle of lag between the compression force and the rotation of the center of gravity. At resonance, this angle is about 90.degree.. Well above resonance (in excess of twice the resonance frequency) the angle of lag approaches 180.degree.. Notwithstanding that the engine speed and resonant frequency of a particular mode may not be exactly equal, the resonance forces may be extreme when the ratio of rotational frequency to natural frequency is in a broad range of from between about 0.5:1 to nearly 2:1.
Upon a blade-off event, the engine ceases normal operation and produces no further power. However, it is typically not desirable to stop rotation of the engine's rotor. If rotation were stopped, the stopped engine would constitute an extreme source of aerodynamic drag. Such drag would be particularly significant in twin-engine aircraft wherein engines are mounted in wing nacelles. This is a common construction for many passenger aircraft. Thus, in such twin-engine aircraft, the combination of drag from the stopped engine and thrust from the remaining engine would produce an excessive yawing moment not easily overcome by the aircraft rudder. Accordingly, the damaged engine is advantageously allowed to rotate, driven by the air flow resulting from the forward velocity of the aircraft in a process called "windmilling". A windmilling engine has significantly less aerodynamic drag than does a completely stopped engine. Under the Extended Range Twin-Engine Operations (ETOPS) rating system, certain aircraft may be required to operate with a windmilling engine for a period of up to 180 minutes. The potentially damaging imbalance forces are transmitted from the windmilling rotor through the bearings to the support frame. To remain windmilling, the engine must resist damage such as bearing seizure for at least the rated ETOPS period. The engine is also preferably configured to avoid catastrophic damage to the support frame which might permit the engine to detach from the aircraft or damage the wing. One approach is to make the bearings and support frame strong enough to withstand the initial imbalance forces until the engine can be safely shut down and allowed to achieve its windmilling speed. Unfortunately, such strengthening of the bearings and support frame adds undesirable weight and bulk to the engine and aircraft.
One possible way to minimize the weight and bulk of the bearings and support frame and also protect the bearings from seizure is to support the rotor on the frame with a support arrangement having a capability to radially constrain the rotor which is abruptly relaxed (or completely defeated) upon being subjected to a radial force in excess of a predetermined value. Once the radial constraint capability is relaxed, the rotor is free to rotate about a rotational axis passing through, or at least closer to its displaced center of gravity. As a result, the transmission of imbalance forces to the support frame is minimized so that its weight and bulk can be correspondingly reduced. In practice, this is achieved by fusibly mounting the bearing which is proximate to the engine fan. When the radial force transmitted through the bearing exceeds a threshold, the bearing at least radially decouples from either of the rotor or the support frame thereby reducing the resistance to local radial displacement of the rotor from the engine axis at least within a broadened range. For example, fusing (release) of the rotor support system could allow radial excursions of up to an inch while, prior to fusing, radial movement is constrained to well under 1/10 inch with respect to the engine axis. A wide variety of structures may accomplish this goal. By way of non-limiting example, fusibly mounted bearings are commonly seen on engines such as the PW305 of Pratt & Whitney Canada Inc. and the TRENT 500 and RB211 of Rolls-Royce plc. Other configurations are also possible such as that shown in U.S. Pat. No. 5,791,789, the disclosure of which is incorporated by reference herein in its entirety.
Immediately upon occurrence of the blade-off event, the engine is turning at an initial operating speed (for example, at its cruise speed), which is in the vicinity of but typically lower than key natural frequencies of the engine as described above (namely the "fan bounce" frequency). In the absence of fusing of the rotor support system, the rotor would go through a spool-down process before entering a steady state condition wherein the phase angle between the imbalance forces and the rotor deflection would be nearly zero as the engine speed decayed from the cruise speed to the windmilling speed. However, the imbalance forces at the beginning of spool-down may be excessive given the relatively high initial speed (e.g., a cruise speed of 2000-2500 rpm) since such forces are proportional to the square of the speed.
It is known to utilize fusible rotor support systems to prevent the high speed forces from being transferred from the shaft to the support structure. Accordingly, there is provided a fusible mount/support (hereinafter "bearing support") coupling the bearing to either the shaft or the non-rotating support structure. The threshold strength of the fusible bearing support may be set to fuse (release) during the initial transient response. Upon release, the natural frequency of the rotor drops dramatically. For example, it may drop to somewhere between about 1/5 and 1/2 of the rotor's initial natural frequency. Thus, upon release, there will be a second transient response as the rotor transitions from conditions associated with the initial natural frequency to those associated with the reduced natural frequency. At the beginning of that second transient response the ratio of engine speed to the reduced natural frequency is well over 2:1 (a condition associated with a phase angle between the imbalance forces and the deflection of approximately 180.degree.). During the second transient, the engine spools down to a cruise windmilling engine speed (e.g., about 700 rpm). Subsequently as the aircraft slows for landing, the windmilling speed will similarly slow (for example, to around 300 rpm). During either of these coast down stages the rotor may go through the reduced natural frequency (wherein the phase angle is 90.degree.) and achieve a phase angle close to zero. The engine is still subject to significant radial displacement of the rotor and associated flexing of the shaft.
However at the reduced natural frequency, the imbalance forces even near resonance may be tolerable if the engine is sufficiently robustly constructed and if prolonged operation near resonance is avoided.
The imbalance forces and displacements during the various transitions and thereafter may still subject the engine and aircraft to excessive loading and undue sympathetic vibration.
Thus, it is seen that mere relaxation of the radial constraint on a rotor is not an entirely satisfactory means for accommodating an unbalanced rotor.